Heat-sensitive transfer recording has been practiced by superposing an image-receiving sheet on a heat transfer sheet having a dye layer containing a heat migratable dye provided thereon under a state where the receiving layer surface of the image-receiving sheet is opposed to the dye layer, and heating the heat transfer sheet by a heating means such as a thermal head which is controlled by electrical signals from the back of the heat transfer sheet, thereby transferring the dye in the dye layer into the receiving layer.
However, in the heat-sensitive transfer method of the prior art, it has been difficult to obtain a printed image having good storability and high recording density.
In general, the dye to be used in the dye layer on the heat transfer sheet has been selected in view of such parameters as recording sensitivity, storability, hue, and dye solubility into ink or binder resin. Among these parameters, recording sensitivity, and storability have been known in the art to have great relationships with molecular weight of the dye and sublimation temperature. For example, it has been considered that the recording sensitivity becomes higher as the molecular weight of the dye is smaller and the sublimation temperature is lower, while storability becomes better as the molecular weight of the dye is larger and the sublimation temperature is higher. From such standpoints, in the prior art, in view of the balance between the both, dyes having molecular weights of about 150 to 800, more preferably 350 to 700, have been frequently selected as desirable ones.
However, even among the dyes having molecular weights within the range of 350 to 700, there are fewer dyes than expected which can give rise to good recording sensitivity, and a dye having desirable performance has been selected and used by repeating screening from a large number of candidates of dyes under the present situation.
Also, the recording sensitivity when performing practical heat transfer is also greatly affected by the combination of the heat transfer sheet and the image-receiving sheet. That is, the combination of the dye layer and the receiving layer, for determining what combination of the receiving layer with the dye selected as described above is optimum, one having better recording sensitivity has been selected also by repeating screening from a large number of materials for formation of receiving layer under the present situation.
Thus, in the development of heat-sensitive recording materials of the prior art, the selection criterion according to dye molecular weight for judgement goodness or badness of recording sensitivity is very vague, and no combination of good materials can be obtained before repeating screening for many times as mentioned above and yet performing practically printing, whereby many difficulties have been encountered in development of heat-sensitive recording materials.